During The Walking Dead Season 4 Premiere, many viewers likely noted that Hershel Greene was sporting a new prosthetic leg. No mention was made during the episode of how or where Hershel got his new leg. During the Talking Dead, Walking Dead showrunner Scott Gimple revealed that there was around a six month time gap between the end of Season 3 and beginning of Season 4. It was during this time gap that Hershel acquired his prosthetic leg.
According to the Talking Dead, Hershel's new prosthetic leg "was found on one of the group’s runs." The Talking Dead also revealed an interesting fact about how the scene where Hershel and Rick are talking in the garden was filmed. Hershel's prosthetic leg can be seen between his pants leg and his boot when he bends down to one knee in the garden. According to the Talking Dead, “Two-legged Scott Wilson had to bury his right leg in the ground for this scene.”
The Walking Dead Season 4 airs on Sunday nights at 9 PM ET on AMC.